Physical activity decreases the risk of multiple cancers and is important for obesity prevention, which in turn is important for cancer prevention. Children's active commuting to school (ACS), previously common (42 percent in 1969-1970) is now uncommon (13 percent in 2009). This decline coincided with the obesity epidemic, which disproportionately affects low-income and ethnic minority children. Programs to increase children's moderate- to-vigorous physical activity (MVPA) and lower obesity risk are necessary. The walking school bus (WSB) is an innovative program in which children walk to and from school led by adults, who teach pedestrian safety. The WSB provides additional time for MVPA, and this team's pilot WSB study reported increases to children's ACS and MVPA in the short-term. No randomized controlled trials (RCTs) have reported on long-term (i.e. over one school year) efficacy of WSB programs on children's ACS, MVPA, pedestrian safety, and adiposity. Few studies have included children from disadvantaged socioeconomic or racial/ethnic backgrounds. The Primary Goal is to conduct a cluster RCT to evaluate the long-term efficacy of a WSB program to increase low-income, ethnic minority children's ACS and MVPA. The investigators will take an interdisciplinary approach and use a social ecological model to consider multiple correlates of ACS and levels of analysis: (1) individual-level influences e.g. acculturation and constructs from social cognitive theory, (2) schools, and (3) macro-environmental influences e.g. neighborhood walkability and safety. To achieve the project goals among 3-5th grade children, the Specific Aims include: SA1) To recruit 770 child-parent dyads from 22 elementary schools over 4 years and conduct a cluster RCT to assess the efficacy of a WSB program on children's ACS, MVPA, and BMI z-score over a school-year SA2) To collect and analyze data on individual-, school-, and macro-level influences on changes to children's ACS resulting from the WSB program The Primary Hypotheses to be tested include: H1) The WSB program will increase children's ACS over a school-year. H1a) Parents' outcome expectations and self-efficacy will mediate the relationship between the WSB and changes to children's ACS. H1b) Walkability, safety, and acculturation will moderate changes to children's ACS. H2) The WSB program will increase children's MVPA and decrease BMI z-scores over a school-year. H3) The WSB program will increase school-level pedestrian safety behaviors over a school-year. The proposed research will allow for optimal assessment of changes to children's ACS, test moderators and mediators of children's ACS, evaluate long term changes to MVPA and BMI z-score, and provide crucial planning information for a future community effectiveness trial.